It really depends on the school you're trying out at. At my school, everyone on varsity has at least a running tuck and is working and extremely close to a standing tuck. But at the school across town, many of their varsity cheerleaders don't even have a standing backhandspring.
I know a big part of our score at tryouts is overall appearance. Go into your clinics and tryouts looking your best.. try not to have hair flying all over your face or shorts that don't fit you well. This is an easy thing to do and can help you to look more put together. Also be CONFIDENT and have a good attitude! Throw a nice smile on your face and look happy.. even if your clinic or tryout isn't going perfectly. There's nothing worse than a slumpy, boring cheerleader. haha
Be tight with your motions, practice your dance or cheer or whatever they give you as much as you can so you can go into the day of tryouts as prepared and calm as possible. And if they do give you a cheer, BE LOUD. And throughout your whole tryout look at the judges. Thats a lot of what high school cheerleading is about- being able to interact with an audience and to lead a crowd and make them excited about whats going on. No ones going to be excited about a game if the cheerleaders aren't peppy and excited too.
Good luck at your tryouts!